#include<stdio.h>
#include<math.h>
double dist(double x1,double y1,double x2,double y2){
	int dist=sqrt((x2-x1)*(x2-x1)+(y2-y1)*(y2-y1));
	return dist;
}
int main(){
	double x1,x2,y1,y2;
	scanf("%lf %lf %lf %lf",&x1,&x2,&y1,&y2);
	printf("%lf",dist(x1,x2,y1,y2));
	return 0;
}
